This invention relates to a position error correcting method and an apparatus for an industrial robot, and more particularly to a method and an apparatus suitable for eliminating errors of the position of an industrial robot attributable to the difference between the robot coordinate system relating to the operation of the industrial robot and the absolute coordinate system of the industrial robot on its working site so that the industrial robot can operate with high accuracy.
For the purpose of controlled operation of an industrial robot, it has been the common practice to prepare a series of robot operation data by teaching and to control the operation of the industrial robot on the basis of the operation data acquired by teaching, and importance has thus been set on the repeatability of the same operation according to the operation data provided by teaching rather than the absolute accuracy of operation according to commanded numerical values.
However, with the recent progress of the so-called factory automation (FA) promoting the automation of various factory operations, an attempt has been started in which the teaching period of time requiring the hands of the operator is eliminated from the operating period of time of an industrial robot, and numerical data prepared offline are used for more efficiently operating the industrial robot. In the case of such an attempt, the capability of control of the robot as commanded by the numerical data, that is, the absolute accuracy becomes a matter of primary importance.
Prior art examples proposed for improving the absolute accuracy of an industrial robot include an apparatus in which values of position errors are previously stored to be read out when error correction is to be done, as, for example, disclosed in Japanese Patent Application Laid-open No. 57-76608 entitled "Position Error Correcting Apparatus" and filed by Fujitsu Fanac Corporation in Japan and Japanese Patent Application Laid-open No. 57-75310 entitled "Position Error Detecting Apparatus for Numerically Controlled Machine Tools" and filed also by Fujitsu Fanac Corporation.
As another prior art example, there is a proposal in which the space is divided into a plurality of small regions, and an error function is computed for each of the individual small regions for the purpose of error correction. However, this proposal is defective in that much time and labor are required for the computation, storage, etc. of the error functions.
As a prior art example designed for measurement of the robot coordinate system peculiar to an industrial robot and the absolute coordinate system of the industrial robot on its working site, there is an apparatus for measuring the position of a robot moving on a plane relative to a work plane as, for example, disclosed in Japanese Patent Application Laid-open No. 57-75793 entitled "Robot Relative Position Measuring Apparatus" and filed by Nippon Denki Co., Ltd. in Japan. However, the proposed apparatus cannot measure the spatial (three-dimensional) relative position of the robot.
As a prior art example proposed for detection of an installation error, a mechanism error and an encoder error of an industrial robot, there is a method entitled "Method of Calibration of Coordinate System of Multiarticulated Robot" and reported by S. Kawakami, et al. of Fujitsu Research Institute, Ltd. in "Reports on Scientific Lectures in Spring Meeting of the Society of Precision Machines, 1982, Vol.2, pp. 587-589." However, the proposed method does not include measurement of the error of the arm mounting angle among the parameters of the robot and is not, therefore, applicable to robots including such an error. Further, the proposal does not refer to any procedure for elimination of the adverse effects of the various errors above described after measurement of the errors.